Serenity is the incarnation of the Moon Goddess Serene and the Queen of the Silver Millennium. Her primary responsibilities were safeguarding the Silver Crystal and nurturing her daughter, Princess Serenity, to become a worthy successor. During this era, a war erupted between Earth and the Moon due to the jealousy of Earth's inhabitants over the extended, prosperous lives enjoyed by those in the Moon Kingdom. Despite this conflict, Prince Endymion of Earth chose to defend the Moon, partly because he was in love with Princess Serenity and partly due to his disapproval of his people's actions.
Tragically, Queen Metallia, ruler of the Dark Kingdom, assaulted the Moon, causing widespread destruction. Everyone perished except for Queen Serenity and her advisors, Luna and Artemis. Grieving the loss of her daughter and subjects, Serenity utilized the Silver Crystal to seal away Queen Metallia and send all who had died that day to be reborn in the future. However, using the full power of the Silver Crystal resulted in Queen Serenity's immediate death. Her spirit remained on the Moon, watching over the Earth until Princess Serenity was reborn.
In the anime, Queen Serenity has a minor role. Near the end of the first series, she appears as a sprite to the Sailor Senshi, revealing the history of the Moon Kingdom. This sprite reappears at the beginning of the R series, returning the Silver Crystal and a new transformation brooch to Usagi. After this, she is never seen again.
In the manga, Queen Serenity's role is more significant. Instead of appearing as a sprite, there is a holographic image of her (though she looks identical). She also appears after Usagi defeats Queen Metallia, asking her to take her place as Queen of the Moon (which Usagi refuses). The story becomes more intriguing in the Super S manga, where Usagi experiences flashbacks to her childhood in the Moon Kingdom. It is revealed that Queen Neherenia, an evil queen, was Queen Serenity's sister, and both originally came from another part of the galaxy before arriving at the Moon. When Princess Serenity was born, Neherenia tried to persuade Queen Serenity to join her in her wicked plans. Upon refusal, Neherenia cursed the baby princess and attempted to claim victory but was defeated when Queen Serenity sealed her in a mirror.
When the princess was around four or five years old, she began having nightmares predicting the outcome of the curse. It was then that Queen Serenity gave Usagi the most crucial advice she would receive throughout the series: "We all have a star in our heart. Darkness and light are always right next to each other. Show a little fear or shed a tear, and darkness will grow, attacking and consuming the light. Serenity, you must always keep the star in your heart shining brightly to defeat dark and evil souls. That is your most important mission, Serenity..."
